After Dr. Fu finished his postdoctoral studies with Dr. James Darnell at Rockefeller University, he started as a principal investigator in January 1992 at the Mt. Sinai School of Medicine in New York. Since last September he has been appointed as a member of the faculty in the Department of Pathology, Yale University. Yale has provided him with an excellent environment. His laboratory has established collaborations in laboratories with Drs. David Stern, Richard Flavell and Jordan Pober, etc., who are well established in cancer biology and immunology. The scientific communication and research at Yale Medical School is one of the best in the world. For his career goals, Dr. Fu has been a productive research scientist since his graduate studies in Dr. James Manley's laboratory at Columbia University, where he was the first to identify the alternative splicing factor, ASF. When he was a postdoctoral fellow with Dr. Darnell, he purified the key interferon- stimulated gene factor, ISGF3, and cloned p91 and p113, the first two members of the STAT factors. As an independent principal investigator, Dr. Fu made the key discoveries on a fundamental signaling pathway linking cell surface receptors to transcription factors. His laboratory first showed that a new class of transcription factors, termed STAT (signal transducer and activator of transcription) contained the SH2 (Src homology region 2), critical domain in tyrosine kinase-mediated signal transduction, and presented the earliest evidence that STAT directly interacts with cell surface receptors (Fu 1992; Fu and Zhang 1993). A novel direct signaling model has been established from this work. This pathway was first discovered in the interferon system and it was shown that activation of STAT factors is a general mechanism to induce gene expression by many tyrosine kinase-associated cytokine receptors. In the next five years Dr. Fu plans to continue studies on STAT factors, focusing on the detailed mechanism of STAT activation, specificities of different STAT factors, and roles of tyrosine kinase/STAT pathway in cancers. His long-term career goal is to reveal the basic mechanism of signal transduction, especially interactions and regulation of different signaling pathways at the cellular level as well as at the level of development. He will apply his knowledge of signal transduction to the treatment of cancer and other human diseases. In this application Dr. Fu proposes the following studies: 1) to reveal the detailed molecular mechanism of activation of STAT by IFNg. He will characterize the IFNg- induced different STAT 91 transcription complexes. This includes purification, characterization and cloning of cytoplasmic co-factors and a possible nuclear DNA-binding factor in the complex. The roles of these possible co-factors in maintaining the specificity of signaling will be determined. Dr. Fu believes that IFNg-induced transcriptional activation and DNA-binding specificities of different STAT complexes are the keys to understanding the important functions of IFNg immunoresponses. 2) to determine the functional roles of p91 in breast cancer cells, especially the cells which overexpress growth factors or their receptors. Phosphorylation states, cellular localization, DNA- binding activities and possible partners of p91 will be analyzed. To reveal the molecular mechanisms of the constitutive activation of p91 by growth factors in some cancer cells.